WordPress is open source web software that you can install on your web server to create your website, blog, community or network. WordPress started out as a tool for blogging, but has evolved into a full-fledged Content Management System (CMS), capable of powering websites, networks and communities.
One of the principal advantages of WordPress is that you are in control. Installation is very simple, as is the configuration. Unlike other software programs, there are not a million files to chmod nor are there dozens of templates to edit just to get your site set up and looking the way you want.
Also, Blog pages in WordPress are generated on the fly whenever a page is requested, so you do not have multiple archive pages clogging up your web space. Waiting for pages to rebuild is a thing of the past because template changes are made in scant seconds.
As seen on WordPress FAQ – from http://codex.wordpress.org/FAQ_About_WordPress.